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Confused(HUGE misunderstanding) about potentiometer (HS physics)

  1. Apr 25, 2009 #1
    Hi everyone. I am extremely confused about how a potentiometer works.

    In my notes, this is the diagram.

    http://img25.imageshack.us/img25/5836/batteryu.jpg [Broken]

    In the branch loop, there is no current. I know this is correct, but Im confused by it. Assuming the current flows clockwise (arbitrary) in the branched circuit,
    V(Of branch battery)=Pd across the length the wires are attached to. (By voltage law)

    Shouldnt there be a current or something? Why is there no current? I know it is a terrible misunderstanding......

    And also what practical use does a potentiometer in this case have? Is it just to see what pd a wire has?

    Thanks a lot for your answers
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 25, 2009 #2

    berkeman

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    Staff: Mentor

    No wonder you are confused. That diagram has nothing to do with a potentiometer, as far as I can see. A pot is a variable voltage divider. The sum of the two divider resistances is constant, and the center tap position determines the divider ratio.

    I'll dig up a good reference for you here in a sec...
     
  4. Apr 25, 2009 #3

    berkeman

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    Staff: Mentor

    Whew. Took a bit of searching, but finally found this link:

    http://www.bourns.com/pdfs/trmrpmr.pdf [Broken]

    It's an excellent tutorial on potentiometers by Bourns, one of the larger manufacturer of pots. It's a classic, and filled with great and useful info.

    BTW, the PF thread that I found the link in is also very useful. Check it out when you have tiime to read a long (but helpful) thread:

    https://www.physicsforums.com/showthread.php?t=178516

    .
     
    Last edited by a moderator: May 4, 2017
  5. Apr 25, 2009 #4
    Whoa, thats a huge amount of info right there. Thanks. But what I dont understand is that you said it is not a potentiometer. Why??? Thats the diagram my school printed in the notes. It is just that I dont understand why is there no current.
    Because of voltage law and assuming current flows anticlockwise,
    Total EMF=Total PD
    EMF=I (R [A-B])
    Shouldnt there be a current?
     
  6. Apr 25, 2009 #5

    berkeman

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    Staff: Mentor

    A potentiometer circuit diagram will show a coupled pair of resistors ( or just a pot symbol ). The voltage sources are different and ancillary to the application.

    Compare and contrast the circuit diagram you posted versus the diagrams in the very reputable App Note from Bourns. Which do you think is correct?
     
  7. Apr 25, 2009 #6

    Borek

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    Staff: Mentor

    Perhaps red wire should be drawn as /\/\/\/\/\/\/\/\ :smile:
     
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